1) Field of the Invention
A polyester with a carbon black content of up to 20% by weight, based on the polyester, is condensed, after transesterification of DMT with diol or after esterification of terephthalic acid with diol, with the aid of a titanium-based catalyst. The carbon black is added in the form of a carbon black dispersion together with the polycondensation catalyst to the transesterification product or esterification product. The carbon black dispersion comprises gas black or furnace black and a dispersing agent, and these are dispersed in diol. The diameter of the carbon black particles is below that of carbon black particles in a polyester using an antimony compound as catalyst.
2) Prior Art
Polyesters have gained major importance in very many fields of application. In particular, saturated polyesters are used for producing fiber materials, and they are also widely used for producing moldings, e.g. for beverage bottles. Very high requirements are placed upon the quality of the polyesters for satisfactory processing of these polyesters through extrusion processes, and for the further use of the extrudates, e.g. in the textile industry or the beverage industry. A particular requirement is that the processing properties and performance of polyester grades for particular processes are similar within very narrow limits. For production of microfilaments with titer below 1 dtex there can be no major additives present in the polyester, since these additives cause filament breakages during spinning.
Polyesters are usually prepared by esterifying aromatic dicarboxylic acids or transesterifying lower-aliphatic esters of aromatic dicarboxylic acids, using aliphatic diols, and then polycondensing until the molecular weight required for the planned use has been achieved.
Transesterification takes place in the presence of transesterification catalysts, which have to be deactivated after termination of the transesterification through addition of complexing agents. Complexing agents are phosphorus-containing compounds, such as phosphoric acid, phosphorous acid, and/or phosphonic acids, or derivatives of the same. After esterification or transesterification, the polycondensation takes place in the presence of a polycondensation catalyst, and antimony compounds, specifically antimony trioxide, have been widely adopted for large-scale industrial technology. It is known that titanium compounds can be used instead of antimony compounds as polycondensation catalysts for PET production.
For example, EP-A 0 699 700 discloses a thermally stable antimony-free polyester using a titanium compound as polycondensation catalyst. Also it discloses a transesterification catalyst metal in the form of catalytically inactive complexes with a phosphorus-containing compound or with a derivative of the same, and also cobalt, some of which is present in the form of catalytically inactive complexes with the phosphorus-containing compound. Additionally, an optical brightener may be used.
U.S. Pat. No. 4,476,272 discloses a polyester of high clarity and low haze, comprising a very low proportion of infra-red-absorbant material, e.g. carbon black. This polyester is used for producing preforms for bottles, and the carbon black used for this purpose has average particle sizes in the range from 10 to 500 nm, but the amount of the carbon black employed is less than 10 ppm, based on the weight of the polyester. The risk of agglomeration of these carbon particles yielding larger particles is very slight since the proportion of carbon black particles is very low.
If, for example, a polyester with carbon black content of 20% by weight, based on the total weight of the polyester, is produced it leads to agglomeration of the fine carbon black particles with particles sizes of from 50 to 500 nm in the admixed carbon black dispersion, to give larger carbon black particles with diameter in the range above 10 μm. Surprisingly, it has been found that the diameter of these agglomerated carbon black particles is dependent on the selected polycondensation catalyst, and it has become apparent that the antimony trioxide catalyst Sb2O3 results in relatively large agglomerated carbon black particles after polycondensation, and these increase the risk of filament breakages in the filaments spun from the polyester.
If microfilaments with titer below 1 dtex are spun from a polyester, the polyester may comprise only very small carbon black particles if filament breakages during spinning are to be avoided.
JP-A 72-26106 has disclosed the improvement of the distributions of carbon black in an antimony-containing polyester by adding fatty acid salt(s) of calcium, of magnesium, of strontium, or of barium. The polyester comprises Sb2O3, calcium acetate monohydrate Ca(OAc)2.H2O, carbon black, and sodium lignosulfonate as dispersing agent.
JP-A 87-144831 describes an antimony-containing polyester which comprises an ethylene-glycol-dispersed carbon black with average particle size of from 0.01 to 1.0 μm, and vinylpyrrolidone-vinyl acetate added as dispersing agent. The polyester extruded in the form of a film has a surface which is smoother than that of a polyester without dispersing agent.
Color dispersions are known from JP-A 70-120212 for the coloring of polyester, and are composed of carbon black and poly(vinylpyrrolidone) as dispersing agent, in each case dispersed in ethylene glycol.
JP-A 73-65445 describes a dispersion made from carbon black and copper phthalocyanine in ethylene glycol, the dispersion being added to an antimony-containing polyester.
The prior art prefers the use of an antimony compound, such as Sb2O3, as polycondensation catalyst together with a carbon black dispersion. A commonly used means of reducing the agglomeration of carbon black particles to a low level or preventing the same where possible, requires in the use of dispersing agents.
It is therefore an object of the present invention to provide a polyester with high carbon black content in which the agglomerated carbon black particles present are markedly smaller than in the prior art.